REACTIONS OF CRAYFISHES. 183 



ment terminated. All the specimens of immunis were smaller 

 in size than the smallest specimen of diogenes. From this data 

 it seems the four species are susceptible to high concentrations 

 of carbon dioxide in the following order: virilis, propinquus, 

 diogenes and immunis. 



V. THE REACTION AND MODIFICATION IN GRADIENTS. 



The reaction of crayfishes in gradients is shown in Tables I. 

 and II. Table I. for carbon dioxide, II. for acetic and hydro- 

 chloric acids, are arranged in order of sharpness of the gradient, 

 i. e., the difference between the concentrations of acid at the two 

 ends. The four species are grouped separately in each table. 

 Reaction is shown by the time preference for one end or the other, 

 by turnings and by crossings of the center. There is also tabu- 

 lated the modification of behavior by turnings accompanied by 

 backing. The backing reaction is not indicated in the ratings 

 (Table III.), which is the numerical expression of avoidance of 

 ends when turnings and time spent in the halves of the tank 

 are considered. The ratio of the increased concentration of the 

 acid of the high end over that of the low end, or Weber's ratio, is 

 tabulated for comparison with the difference of concentration 

 of the acid at the two ends. 



The crayfishes, when passing into the high concentration end 

 of the tank, gave certain definite avoiding reactions, when not 

 too much affected by the presence of the acid. Of these the 

 reactions recordable in graphs are (i) turning upon encountering 

 gradient, either the first time or after one or more invasions and 

 (2) reactions which cannot be recorded graphically are turning 

 accompanied by backing and crawling on the screen, out of the 

 water, or attempting to crawl on the sides of the tank. Record- 

 able reactions were in the most cases rhythmic and represented 

 a rapid modification of behavior. Reactions of the first class 

 are grouped in one column of the table, but where backing 

 accompanied turning separate mention is made of this fact. Of 

 the reactions that could not be graphically recorded, that of 

 crawling on the screen is least definite as an avoiding reaction 

 since it was noted in the controls also, though less than in the 

 experiments. This reaction is probably due to thigmotactic 



